This invention relates to a digital signal recording circuit for recording digital video or audio signals on a magnetic tape using a rotary head.
Recently, attempts have been made in a digital video tape recorder or a digital audio tape recorder to shorten the wavelength of data signals and to increase the speed and frequency for realizing large capacity and high recording density and speed. Consequently, for recording data signals on a magnetic tape, it becomes necessary to improve precision and performance of a rotary transformer employed for transmitting data signals to the rotary head.
The rotary transformer transmits data signals to the rotary head while it is electrically coupled to but mechanically separated from the rotary head. The closer the coupling coefficient to unity, the more efficiently data signals can be transmitted from a primary coil as a stationary side to a secondary coil as a rotary side.
Among digital signal recording/ reproducing apparatus, transmitting data to the rotary head using such a rotary transformer, a typical conventional digital signal recording circuit will be explained by referring to FIG. 1.
To an input terminal 51 of the conventional digital signal recording circuit are fed data signals via a digital signal processing system of a preceding stage, not shown. These data signals are supplied to a data terminal D of a D-flipflop circuit 53, a clock input terminal CL which is fed via an input terminal 52 with clock signals produced by the digital signal processing system. The data signals thus supplied via an input terminal 51 from the digital signal processing system are synchronized and wave-shaped by the D-flipflop circuit 53 and thence fed to the primary coil of the rotary transformer 54.
The rotary transformer 54 transmits the data signals supplied to its primary coil from the D-flipflop circuit 53 to its rotating secondary coil positioned with a minor gap from the primary coil. The signals transmitted to the secondary coil are supplied via a differential amplifier 55 to bi-level means, such as a slicer 56, and thereby turned into bi-level signals, which are then supplied from the slicer 56 to a magnetic head 58 via an amplifier circuit 57. The magnetic head 58 records the bi-level data signals on the magnetic tape.
However, it is not possible with the rotary transformer 54 to transmit DC and low-frequency components of the signals supplied to its primary coil. That is, the low-frequency components are lost from the transmitted data signals since the rotary transformer 54 cuts off the low-frequency components of the transmitted data signals.
The low-range cut-off characteristics of the rotary transformer are attributable to the inductance of the rotary transformer and the resistance of other neighboring circuits, such as a signal source. FIG. 2 shows frequency versus amplitude characteristics of the rotary transformer shown by an equivalent circuit of FIG. 4. In the equivalent circuit of FIG. 4, C.sub.p1 =5 pF, L.sub.0 =10 .mu.H, k (=coupling coefficient)=0.98, C.sub.p2 =5 pF and R.sub.L =100 ohms.
If the signal source resistance is set to 100 or 200 ohms, the signal amplitude is acutely lowered for the frequencies of not higher than 1 MHz. For example, if the signal source resistance R is 100 ohms, the equivalent circuit shown in FIG. 4 is equivalent in operation to a first-order high-pass filter (HPF) having a cut-off frequency fc equal to 1 MHz. The result is that, even if the voltage waveform of the signal source is a distortion-free rectangular wave, the voltage waveform of the output signal appearing at both ends of a load resistor R.sub.L is susceptible to so-called sag. If the signal source resistance R is set to 5 ohms, the cut-off frequency fc can be set to 0.3 MHz or thereabouts. It is, however, still not possible to transmit DC components. On the other hand, if the signal source resistance R is set to 5 ohms, delay characteristics are deteriorated, as shown in FIG. 3.
In order to cope with interruption of the low-frequency signals, attempts have been made to constitute an inverse circuit by a network comprising coils and capacitors. However, such a network has not been put to practical use because it is, however, not possible with such an inverse circuit to transmit the DC components and, besides, such an inverse circuit leads to an increased circuit scale.
There has also been known a method of narrowing the range of the data signals by phase modulation for producing data signals suited to the rotary transformer characteristics and transmitting these narrow range data signals. However, such a method cannot be adapted to higher transmission rates because the upper limit of the transmission range required in such a case is doubled and hence the transmitted signals are lowered in amplitude by high-range interruption and the waveform is susceptible to rounding.
Thus there lacks up to now a method effective in compensating for low-range cut-off characteristics of the rotary transformer. The result is that, if a signal free of transitions is supplied to a system employing the rotary transformer having the above-mentioned low-range cut-off characteristics, the low-range amplitude is attenuated exponentially. Should the amplitude of the low-range components become lower than the noise level or the offset level of the input to a differential amplifier circuit, correct data signals cease to appear at outputs of the bi-level means or the wave-shaping means, resulting in incorrect timing of the inversion of the recording magnetization. In the worst case, data signal dropout is incurred and affects the patterns of recording magnetization to render it impossible to record correct data signals.